Swift And CoreData With Custom Class As Transformable Object

I am trying to use a custom class with swift and CoreData as a transformable object. I have been wasting hours trying to figure it out but cannot. I keep getting two errors: Property cannot be marked @NSManaged because its type cannot be represented in Objective-C and Property cannot be declared public because its type uses an internal type.
Help is greatly appreciated. Thanks.

I have a CoreData object named User. I also have an object named Site.
I don't want the Site object to be a CoreData object because the majority of the times that I create a Site object I don't want it added to the CoreData context.

Below you will see "primarySite" of type "Site" and "sites" which is an array of type "Site". These two lines give the same two errors I listed above.

Data Model

User+CoreDataClass

import Foundation
import CoreData

public class User: NSManagedObject {
}

User+CoreDataProperties

import Foundation
import CoreData

extension User {
    @nonobjc public class func fetchRequest() -> NSFetchRequest<User> {
        return NSFetchRequest<User>(entityName: "User");
    }

    @NSManaged public var firstName: String?
    @NSManaged public var lastName: String?
    @NSManaged public var role: String?
    @NSManaged public var primarySite: Site
    @NSManaged public var sites: [Site]
}

Site

import UIKit

class Site: NSCoding {
    let identifier: Int32
    let name: String
    let note: String

    init(with values: Dictionary<String,Any>) {
        identifier = Int32(values["identifier"] as! String)!
        name = values["name"] as! String
        note = values["note"] as! String
    }

    required convenience init(coder decoder: NSCoder) {
        var siteValues = [String:Any]()
        siteValues["identifier"] = decoder.decodeObject(forKey: "identifier") as! String
        siteValues["name"] = decoder.decodeObject(forKey: "name") as! String
        siteValues["note"] = decoder.decodeObject(forKey: "note") as!
        self.init(with: siteValues)
    }

     func encode(with coder: NSCoder) {
        coder.encode(String(self.identifier), forKey: "identifier")
        coder.encode(self.name, forKey: "name")
        coder.encode(self.note, forKey: "note")
    }
}

I figured out how to resolve both errors.

1) "Property cannot be marked @NSManaged because its type cannot be represented in Objective-C"
     Thanks to @Tom's answer I resolve this error by making Site a subclass of NSObject.

class Site: NSObject, NSCoding {

2) "Property cannot be declared public because its type uses an internal type"
     I resolve this error by making adding "public" before the class, and the encoder and decoder functions.

public class Site: NSObject, NSCoding {
...
public func encode(with coder: NSCoder) {...}
public required convenience init(coder decoder: NSCoder) {...}